- Haskell-Cafe - Haskell.org

Gtk2hs
by Andrew Coppin 15 Mar '08

15 Mar '08

Just a short one... gtk2hs won't build on my [Linux] laptop. What's the best channel for seeking help with this?

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Type system
by Andrew Coppin 15 Mar '08

15 Mar '08

Haskell has an expressive and powerful type system - which I love. It also has a seemingly endless list of weird and obscure type system extensions. And there are various things you can do in Haskell which *require* some pretty serious type system hackery. And yet, none of this happens in any other programming language I've met. Take Java for example. In Java, a "type" is just an identifier. It has no further structure than that. It does have the added semantics that it refers to that class *or any subclass*, but that's about it. Even taking an extreme example like Eiffel [which supports generics], complex types are extremely rare. And yet they commonly pop up in Haskell. Can anybody put their finger on precisely why that is? Is it because Haskell is used by more PhDs? Is it because Haskell actually allows you to implement constructs that are impossible in other languages? Is it because Haskell really provides greater type safety? Is it something else? Not trying to suggest that either system is "superior", I'm just curios as to the origin of the difference...

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thanks for your feedback concerning tech documentation.
by michael＠schmong.org 15 Mar '08

15 Mar '08

Thanks. I was encouraged by this response I got. I'm ready to go. Since I'm trapped in the space-time continuum like most people, I can't do it all at once. I would like to. Anything that supports haskell is okay by me. My first area of interest is HAppS. I wrote some e-mail to them yesterday, but have yet to hear anything. Should I belt out some e-mail to specific people? By the looks of things, the traffic on that list is pretty slow right now. Michael Litchard

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Re: [Haskell-cafe] Gtk2hs
by Andrew Coppin 15 Mar '08

15 Mar '08

Don Stewart wrote: > andrewcoppin: > >> Just a short one... gtk2hs won't build on my [Linux] laptop. What's the >> best channel for seeking help with this? >> > > Discuss it on the gtk2hs list, with a full error log. > Thanks. I'll go look at that. (Who knows, maybe somebody already solved this one...)

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Ackermann Function Memoization, GHC Weird Output or Bug?
by Donnie Jones 15 Mar '08

15 Mar '08

Hello, I'm learning Haskell, so I was attempting memoization based upon the Fibonacci examples but for the Ackermann function. In my tests, I found what seems to be truncated output. See my comments at the end of the code for the test cases and output. ### Begin Code ### module Main where import Data.Array main = do let m = 3 n = 1 a = ackermann_mem m n putStrLn("Ackermann-mem " ++ show m ++ " " ++ show n ++ " = " ++ show a) -- Functions. -- Based upon examples from: -- http://reddit.com/r/programming/info/16ofr/comments) <http://reddit.com/r/programming/info/16ofr/comments%29> tabulate bounds f = array bounds [(i, f i) | i <- range bounds] dp bounds f = (memo!) where memo = tabulate bounds (f (memo!)) -- Trying to apply memoization function to Ackermann. ackermann_mem m n = dp ((0,0), (30, 1000)) ack (m, n) where ack rec (0, n) = n + 1 ack rec (m, 0) = rec (m - 1, 1) ack rec (m, n) = rec (m - 1, rec (m, n - 1)) {- Test cases: ackermann_mem 4 1 = 533 -- when using (30, 1000) as upper bound. ackermann_mem 4 1 = 5533 -- when using (30, 10000) as upper bound. ackermann_mem 4 1 = 65533 -- when using (30, 100000) as upper bound. <--- correct answer! -} ### End Code ### It seems if I don't choose an upper bound pair for (m,n) that is large enough I get truncated output for the answer, instead of GHC giving me an array index exception... This behavior seems very odd to me, can someone explain? Or is this a bug? Thank you. __ Donnie Jones

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File I/O question
by Andrew Coppin 15 Mar '08

15 Mar '08

Hi Cafe. There's good news and there's bad news. The bad news is... I'm back. [Did I miss anything good?] The good news is... I have an actual question to ask as well. When I write to a file using System.IO, the file is locked for exclusive access. I gather this is as specified in the Haskell Report. Which is nice, but... I'd actually prefer the file to *not* be locked. Anybody know how to do that?

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Space leak - help needed
by Krzysztof Kościuszkiewicz 15 Mar '08

15 Mar '08

Dear Haskellers, Another story from an (almost) happy Haskell user that finds himself overwhelmed by laziness/space leaks. I'm trying to parse a large file (>600MB) with a single S-expression like structure. With the help of ByteStrings I'm down to 4min processing time in constant space. However, when I try to wrap the parse results in a data structure, the heap blows up - even though I never actually inspect the structure being built! This bugs me, so I come here looking for answers. Parser follows: > module Main where > > import qualified Data.ByteString.Lazy.Char8 as B > import Text.ParserCombinators.Parsec > import Text.ParserCombinators.Parsec.Pos > import System.Environment > import System.Exit > import qualified Data.Map as M > import Lexer > > type XdlParser a = GenParser Token XdlState a > > -- Parser state > type XdlState = Counts > > type Counts = M.Map Count Integer > > data Count = ListCount | SymbolCount > deriving (Eq, Ord, Show) > > emptyXdlState = M.empty > > incCount :: Count -> (Counts -> Counts) > incCount c = M.insertWith' (+) c 1 > > -- handling tokens > myToken :: (Token -> Maybe a) -> XdlParser a > myToken test = token showTok posTok testTok > where > showTok = show > posTok = const (initialPos "") > testTok = test > > -- Syntax of expressions > data Exp = Sym !B.ByteString | List ![Exp] > deriving (Eq, Show) > > expr = list <|> symbol > > rparen = myToken $ \t -> case t of > RParen -> Just () > other -> Nothing > > lparen = myToken $ \t -> case t of > LParen -> Just () > other -> Nothing > > name = myToken $ \t -> case t of > Name n -> Just n > other -> Nothing > > list = do > updateState $ incCount ListCount > lparen > xs <- many1 expr > rparen > return () > -- return $! (List xs) > > symbol = do > updateState $ incCount SymbolCount > name >> return () > -- Sym `fmap` name > > -- Top level parser > top :: XdlParser XdlState > top = do > l <- many1 list > eof > getState > > main = do > args <- getArgs > case args of > [fname] -> do > text <- B.readFile fname > let result = runParser top emptyXdlState fname (tokenize text) > putStrLn $ either show show result > _ -> putStrLn "usage: parse filename" >> exitFailure And the Lexer: > module Lexer (Token(..), tokenize) where > > import qualified Data.ByteString.Lazy.Char8 as B > import Control.Monad > import Data.Char > import Data.List > import System.Environment > import System.Exit > > data Token = LParen > | RParen > | Name B.ByteString > deriving (Ord, Eq, Show) > > type Input = B.ByteString > > -- Processor returns Nothing if it can't process the Input > type Processor = Input -> Maybe ([Token], Input) > > -- Tokenize ends the list when all processors return Nothing > tokenize :: Input -> [Token] > tokenize = concat . unfoldr processAll > where > processors = [doSpaces, doComment, doParens, doName] > processAll :: Processor > processAll bs = if B.null bs > then Nothing > else foldr mminus Nothing $ map ($ bs) processors > mminus a@(Just _) _ = a > mminus Nothing b = b > > doSpaces :: Processor > doSpaces bs = > if B.null sp > then Nothing > else Just ([], nsp) > where > (sp, nsp) = B.span isSpace bs > > doComment :: Processor > doComment bs = > if B.pack "# " `B.isPrefixOf` bs > then Just ([], B.dropWhile (/= '\n') bs) > else Nothing > > doParens :: Processor > doParens bs = case B.head bs of > '(' -> Just ([LParen], B.tail bs) > ')' -> Just ([RParen], B.tail bs) > _ -> Nothing > > doName :: Processor > doName bs = > if B.null nsp > then Nothing > else Just ([Name nsp], sp) > where > (nsp, sp) = B.span (not . isRest) bs > isRest c = isSpace c || c == ')' || c == '(' Regards, -- Krzysztof Kościuszkiewicz Skype: dr.vee, Gadu: 111851, Jabber: kokr(a)jabberpl.org "Simplicity is the ultimate sophistication" -- Leonardo da Vinci

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FFI newbie question
by Verma Anurag-VNF673 14 Mar '08

14 Mar '08

I am trying to figure out how to pass array of String (char **) from C to Haskell? I have read the FFI examples, but most of them are centered on calling C from Haskell. I have read in the mailing list, it is rare to call Haskell from C, but my requirement is such that I am going to write Haskell library that needs to be called from C. Also, I read that the linking in FFI needs to be done through ghc. However, I have an existing Make system that uses gcc to do the linking, so what extra steps I need to do make gcc do the linking instead? Anurag

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A question about "monad laws"
by Deokjae Lee 14 Mar '08

14 Mar '08

Tutorials about monad mention the "monad axioms" or "monad laws". The tutorial "All About Monads" says that "It is up to the programmer to ensure that any Monad instance he creates satisfies the monad laws". The following is one of the laws. (x >>= f) >>= g == x >>= (\v -> f v >>= g) However, this seems to me a kind of mathematical identity. If it is mathematical identity, a programmer need not care about this law to implement a monad. Can anyone give me an example implementation of monad that violate this law ?

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Problem making a program in ghc
by Maverick 14 Mar '08

14 Mar '08

Hi, I have a problem when I am trying to use a binary file generated by ghc make in Windows. Ihave writed an application that parses xml files and returns a result(using HaXml), this is published as a service, I implemented a server(withSocketsDo), the server listen the port 5760 in a forever fashion,when a message arrives, then the program creates a new thread (forkIO)for attend the request. This works fine when I run it from ghci, when I make the program and run the binary (in Windows), the requests arrive to the program but I think that the response can´t be sentbecause the front end can´t receive the response (I am calling theservice from a .Net web application), I have a log that confirms that the response arrives correctly. I use make like this: ghc --make -package HaXml Main.hs HermesService.hs ..... What could be the problem? I am using HaXml 1.13.2 with Ghc 6.6.1, thanks in advance. Alvaro Sejas UMSS Cochabamba - Bolivia ______________________________________________ Enviado desde Correo Yahoo! El buzón de correo sin límite de almacenamiento. http://es.docs.yahoo.com/mail/overview/index.html

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Naming as infection
by Greg Meredith 14 Mar '08

14 Mar '08

All, Here <http://biosimilarity.blogspot.com/2008/03/naming-as-dialectic.html> is a deliberately provocative posting (with running code and a shameless plug for BNFC) on the process of introducing naming and name management into the design of data structures. Comments greatly appreciated. Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

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Re: Some clarity please! (was Re: [Haskell-cafe] Re: (flawed?) benchmark : sort)
by Conor McBride 14 Mar '08

14 Mar '08

Hi On 13 Mar 2008, at 23:42, kahl(a)cas.mcmaster.ca wrote: > Conor McBride <conor(a)strictlypositive.org> responded to my comment: > >>> (mapMonotonic should of course be removed, too, >>> or specified to fail (preferably in some MonadZero) >>> if the precondition is violated, >>> which should still be implementable in linear time.) >> >> What's wrong with mapMonotonic that isn't wrong >> with, say, sortBy?, or Eq instances for parametrized >> types? > > Prelude> :m + Data.Set > Prelude Data.Set> toAscList $ mapMonotonic (10 -) (fromList [1 .. 5]) > [9,8,7,6,5] > Prelude Data.Set> 5 `member` mapMonotonic (10 -) (fromList [1 .. 5]) > False > > > Something's certainly wrong there! But nothing out of the ordinary: garbage in, garbage out. Happens all the time, even in Haskell. Why pick on mapMonotonic? Prelude Data.List> sortBy (\_ _ -> GT) [1,3,2,5,4] [4,5,2,3,1] Prelude Data.List> sortBy (\_ _ -> GT) [4,5,3,2,1] [1,2,3,5,4] I guess there's a question of what we might call "toxic waste"---junk values other than undefined. I think undefined is bad enough already. So the type system can't express the spec. I don't think we should be casual about that: we should be precise in documentation about the obligations which fall on the programmer. Some dirt is pragmatically necessary: we shouldn't pretend that it ain't so; we shouldn't pretend that dirt is clean. > > >> >> Before we can talk about what == should return, >> can we settle what we mean by = ? > > ``='' is not in the Haskell interface! ;-) No, but "is" is in the human interface! > > Therefore, I talked only about (==). Ah, but you talked about things. Which things? Is one of the things you talked about the same as (==)? the same as (flip (==))? > > The best way to include ``='' seems to be the semantic equality of > P-logic > [Harrison-Kieburtz-2005], which is quite a heavy calibre, > and at least in that paper, classes are not yet included. I expect it's hard work. It's hard work in much better behaved systems. My point is that it's worth it, in order to facilitate more meaningful discussions. All the best Conor

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Re: Some clarity please! (was Re: [Haskell-cafe] Re: (flawed?) benchmark : sort)
by Conor McBride 14 Mar '08

14 Mar '08

Hi folks I'm late into this thread, so apologies if I'm being dim. On 13 Mar 2008, at 16:17, kahl(a)cas.mcmaster.ca wrote: > Adrian Hey <ahey(a)iee.org> wrote: > >> I would ask for any correct Eq instance something like the law: >> (x==y) = True implies x=y (and vice-versa) I wish I knew what = meant here. Did somebody say? I don't think it's totally obvious what equational propositions should mean. Nor do I think it's desirable to consider only those propositions expressible in QuickCheck. If = is reflexive and distinguishes undefined from True, then x = y implies (x == y) = True will be tricky to satisfy for quite a lot of types. What about undefined == undefined or repeat 'x' == repeat 'x' ? For some suitable (slightly subtle) definition of "finite", you might manage x finite and x = y implies (x == y) = True One rather intensional definition of x = y might be "x and y have a common n-step reduct" with respect to some suitable operational semantics. I don't think this is what Adrian had in mind, but it certainly falls foul of Wolfram's objection. > > The easiest counterexample are sets (or finite maps) > provided as an abstract data type (i.e., exported without access to > the > implementation, in particular constructors). > Different kinds of balanced trees typically do not produce the same > representation for the same set constructed by different construction > expressions. This suggests that we should seek to define x = y on a type by type basis, to mean "x and y support the same observations", for some suitable notion of observation, which might depend crucially on what operations are exported from the (notional or actual) module which defines the type. If so, it's clearly crucial to allow some observations which rely on waiting for ever, in order to avoid _|_-induced collapse. Something of the sort should allow this... > > Therefore, (==) on sets will be expected to produce equality of sets, > which will only be an equivalence on set representations. ...but this... > >> which implies (f x == f y) = True (for expression types which are >> instances of Eq). > > This specifies that (==) is a congurence for f, and is in my opinion > the right specification: (==) should be a congurence > on all datatypes with respect to all purely defineable functions. ...is more troublesome. Take f = repeat. Define f = f. I'd hope x == y = True would give us x = y, and that x == y would be defined if at least one of x and y is finite. That implies f x = f y, which should guarantee that f x == f y is not False. > But at least nowadays people occasionally do export functions > that allow access to representation details, [..] > I consider this as an argument to remove showTree from the > interface of > Data.Set, and to either specify toList to produce an ordered list > (replacing toAscList), or to remove it from the interface as well. Perhaps that's a little extreme but I agree with the sentiment. How about designating such abstraction- breaking functions "nosy", in the same way that functions which might break purity are "unsafe". > (mapMonotonic should of course be removed, too, > or specified to fail (preferably in some MonadZero) > if the precondition is violated, > which should still be implementable in linear time.) What's wrong with mapMonotonic that isn't wrong with, say, sortBy?, or Eq instances for parametrized types? > >> but if so would like to appeal to movers and shakers in the >> language definition to please decide exactly what the proper >> interpretation of standard Eq and Ord "class laws" are and in the >> next version of the report give an explanation of these > > Strongly seconded, inserting ``precise'' before ``explanation'' ;-) > > (And I'd expect equivalences and congruences to be accessible > on the basis of standard first-year math...) Before we can talk about what == should return, can we settle what we mean by = ? I think we need to pragmatic about breaking the rules, given suitable documentation and maybe warnings. We should at least aspire to some principles, which means we should try to know what we're talking about and to know what we're doing, even if we don't always do what we're talking about. I'll shut up now. Potatoes to peel Conor

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Video / Slides: Type-driven testing in Haskell (Simon Peyton Jones)
by Robert Hulme 14 Mar '08

14 Mar '08

I was lucky enough to attend a lecture by SPJ yesterday. You're lucky that I taped it and have put it all online :-) http://www.foomongers.org.uk/videos/spj-typedriventestinginhaskell.html If you're a redditer please vote it up on programming.reddit! http://reddit.com/r/programming/info/6byuy/comments/ -Rob -- ----------------------------------------------------- "She's flushing me out with dogs and grenades, and a sincere desire to spend time with me" --Mark Corrigan "That's the way things are these days, let's just put a zip here, a swastika there. Who the hell even knows what these things were used for. Who cares?" --Mark Corrigan http://www.robhulme.com/ http://robhu.livejournal.com/

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Exception handling when using STUArray
by Xiao-Yong Jin 14 Mar '08

14 Mar '08

Dear list, I'm using STUArray in some of my time critical number crunching code. I would like to know some way to catch the exceptions raised in the ST monad, ie. ArrayException. Looking through the Control.Exception module, I understand that those functions can only be used within IO monad. So my question is, what the best way to do exception handling in STUArray is. Plus, it would be helpful if I can also raise an exception in ST monad, but how? Thanks, Xiao-Yong -- c/* __o/* <\ * (__ */\ <

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Bug with GADT in function Patterns?
by Hugo Pacheco 13 Mar '08

13 Mar '08

Hi guys, I have found a bug on the compiler (at least ghc >6.8.2). For some module (yes, the example does nothing at all): *module Test where data Type a where Func :: Type a -> Type b -> Type (a -> b) PF :: Type a -> Type (PF a) data PF a where ID :: PF (a -> a) test :: Type a -> a -> a test (PF (Func _ _)) ID = ID* I get the impossible: *$ ghci Test.hs -fglasgow-exts GHCi, version 6.9.20080303: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling Test ( Test.hs, interpreted ) ghc-6.9.20080303: panic! (the 'impossible' happened) (GHC version 6.9.20080303 for i386-apple-darwin): Coercion.splitCoercionKindOf $co${tc aog} [tv] <pred>t_ao8{tv} [tau] ~ a{tv aob} [sk] -> a{tv aob} [sk] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug* However, the following implementations of *test* compile ok: *test :: Type a -> a -> a test (PF _) ID = ID test :: Type a -> a -> a test (PF (Func _ _)) x = x* It has something to do with mixing different GADTs contructors. Should this be submitted as a bug as it is? Cheers, hugo

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IO () and IO [()]
by Paulo J. Matos 13 Mar '08

13 Mar '08

Hello all, I find it funny that IO () is different from IO [()]. For example, if I define a function to output some lines with mapT, I would do: outputLines :: Int -> IO () outputLines i = mapM (putStrLn . show) (take i $ iterate ((+) 1) 1) However, this is in fact outputLines :: Int -> IO [()] I would like to know if in fact there's any difference in practice between (), [()], i.e. if in practice the difference matters. My first guess is that this is just a consequence of the Haskell type system and so that everything fits this really needs to be like this. Because mapM :: (Monad m) => (a -> m b) -> [a] -> m [b] So I guess that it makes sense that you get IO [()] instead of IO (), and adding an exception just to say that [()] == () isn't good. By the way, as a consequence can you possibly get IO (()) or IO ([()]) and are these all different from each other? Cheers, -- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK Sponsor ECS runners - Action against Hunger: http://www.justgiving.com/ecsrunslikethewind

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An offer to any haskell projects out there.
by michael＠schmong.org 13 Mar '08

13 Mar '08

Hello, My name is Michael Litchard. I'm a techie living in silicon valley, and I want to move into tech writing. I've got the background, now I need a portfolio. I figured the best way to go is to attach myself to some open source projects, and haskell has had my heart for a few years now. I am by no means an expert at haskell. My expertise is writing. So I make this offer. If you need documentation written for your haskell project, I'm your man. Whatever it is, I'll write it or edit it. Thanks for your time. P.S If this was the wrong list, or if anyone has other ideas about how to propogate my proposal, please let me know. Michael

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Reflective capabilities of Haskell (cont'd)
by Martin Hofmann 13 Mar '08

13 Mar '08

I am trying to port a programme written in Maude, which is a reflective language based on rewriting logic ( http://maude.cs.uiuc.edu/ ), to Haskell. I thought using Template Haskell might be a good idea, but I got stuck and now I am wondering if this is really possible in Haskell. Let me give an example of a Maude module defining the function last over a list of Peano numbers. fmod LAST is sorts Peano PeanoList . op 0 : -> Peano [ctor] . op s : Peano -> Peano [ctor] . op [] : -> PeanoList [ctor] . op cons : Peano PeanoList -> PeanoList [ctor] . op last : PeanoList -> Peano . vars X Y : Peano . var Xs : PeanoList . eq last(cons(X,[])) = X . eq last(cons(X,cons(Y,Xs))) = last(cons(Y,Xs)) . endfm So, last(cons(s(0),cons(s(s(0)),cons(s(s(s(0))),[])))) would reduce to s(s(s(0))). The cool thing about Maude is, that terms, modules, ... can be lifted to the meta level. For example: upModule('LAST, false) would yield fmod 'LAST is including 'BOOL . sorts 'Peano ; 'PeanoList . none op '0 : nil -> 'Peano [ctor] . op '`[`] : nil -> 'PeanoList [ctor] . op 'cons : 'Peano 'PeanoList -> 'PeanoList [ctor] . op 'last : 'PeanoList -> 'Peano [none] . op 's : 'Peano -> 'Peano [ctor] . none eq 'last['cons['X:Peano,'`[`].PeanoList]] = 'X:Peano [none] . eq 'last['cons['X:Peano,'cons['Y:Peano,'Xs:PeanoList]]] = last['cons[ 'Y:Peano,'Xs:PeanoList]] [none] . endfm I also have access, e.g. to the defined type constructors. So upOpDecls('LAST,false). yields op '0 : nil -> 'Peano [ctor] . op '`[`] : nil -> 'PeanoList [ctor] . op 'cons : 'Peano 'PeanoList -> 'PeanoList [ctor] . op 'last : 'PeanoList -> 'Peano [none] . op 's : 'Peano -> 'Peano [ctor] . Given an arbitrary function, I have access to its definition, its types and the type constructors, all as ADTs. Is this possible with Template Haskell, or in some other way? Thanks a lot, Martin

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Re: unification
by Christian Maeder 13 Mar '08

13 Mar '08

andrea wrote: >> data Tipo = Var Int | Const String | Tipo :-> Tipo deriving (Show, Eq) >> > But now how can I do to make the set of Const fixed?? > I want just a few base types, like "int" and "string", how could I > define it? your base types could be: Const "String" :: Tipo Const "Int" :: Tipo and the polymorphic type of the identity function could be: Var 1 :-> Var 1 :: Tipo (and with "infixr :->") the type of binary function like (+) Const "Int" :-> Const "Int" :-> Const "Int" Note: the data type does not support type constructors like "List". I recommend from: http://www.haskell.org/haskellwiki/Research_papers/Type_systems Typing Haskell in Haskell The first few pages will be enough (substitution is on page 7) However, the link http://www.cse.ogi.edu/~mpj/pubs/thih.html does not work for me Could someone correct that link? Christian

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floating point operations and representation
by Don Stewart 13 Mar '08

13 Mar '08

> I am under the restriction that I need to write Haskell programs using > Double which mimic existing C/C++ programs or generated data sets, and > get the same answers. (It's silly, but take it as a given > requirement.) If the C programs are using "log2", then I need "log2" > in the Haskell, or else I run the risk of not producing the same > answers. Hey Jacob, Just to make life super simple, I packaged up a binding to the basic math.h library for Doubles. You can find the library here: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cmath For example, Prelude> Foreign.C.Math.Double.log10 5 0.6989700043360189 Prelude> log 5 / log 10 0.6989700043360187 Enjoy! -- Don

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Re: Reflective capabilities of Haskell (cont'd)
by oleg＠okmij.org 13 Mar '08

13 Mar '08

Martin Hofmann wrote: > Thanks a lot, this helps a bit, but access to function bodies is exactly > what I need. Then perhaps you might like the method of reconstructing bodies (of possibly compiled) functions http://okmij.org/ftp/Computation/Generative.html#diff-th in the form of AST -- the template Haskell AST. The reconstructed bodies of functions can be arbitrarily manipulated (e.g., _symbolically_ differentiated or algebraically simplified) and then converted `back' to the compiled code.

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[GSoC] A data parallel physics engine
by Roman Cheplyaka 13 Mar '08

13 Mar '08

I'm looking for interesting project to work on during Google Summer of Code. So I found [1]"A data parallel physics engine" ticket and got excited about it. I'd like to know interested mentors and community opinion about the complexity of such project. I have not very deep knowledge about both NDP and physics engines. Is it feasible to learn all the necessary stuff during these 2 month before SoC starts? 1. http://hackage.haskell.org/trac/summer-of-code/ticket/1535 -- Roman I. Cheplyaka :: http://ro-che.info/ ...being in love is totally punk rock...

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linker question
by Galchin Vasili 13 Mar '08

13 Mar '08

Hello, I have made modifications against an existing Haskell library and installed in my --prefix=$HOME. How do I specify to link against this test .a and not the "global" archive? Regards, vasili

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Java interfacing and ghc-6.8?
by Marc Weber 13 Mar '08

13 Mar '08

Hi, I've read on haskell org about gcjni Haskel/Java VM bridge Lambada (< ghc 6.6.1) Do you know wether any of them can be compiled with ghc-6.8 ? If not does it need much effort to patch them? Sincerly Marc Weber

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Constructing Data.Map from ByteString
by Dave Tapley 13 Mar '08

13 Mar '08

Hi all, I've been plugging away at this all day and some discussion in #haskell has been fruitless. Perhaps you have the inspiration to see what's happening! Concerning this minimal example: http://hpaste.org/6268 It works as required, loading K/V pairs into a Data.Map, the concern is the amount of memory used. Pausing (using a getLine) after the readFile one can see (through 'ps v') that the whole file 'f' is loaded in to memory. However once the Map is created the memory footprint swells to around five times the size. Surely this can't be just overhead from Map? My reasoning was that by using ByteString and getting the whole file into memory a small and linear increase would be seen for Map overhead.. I have tried using both Data.ByteString.Char8 and Data.ByteString.Lazy.Char8 with negligible difference. For a hoot I tried it with String and yes, it's ridiculous :) Cheers, Dave

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Reminder: 4th Haskell Hackathon, April 11-13 in Gothenburg
by Bjorn Bringert 13 Mar '08

13 Mar '08

------------------------------------------------------------------------ Hac4 4th Haskell Hackathon April 11-13, 2008 Gothenburg, Sweden http://haskell.org/haskellwiki/Hac4 Sponsored by Credit Suisse and Galois. ------------------------------------------------------------------------ This is a reminder to register for the 4th Haskell Hackathon. The event will be held over 3 days, April 11-13, at Chalmers University of Technology in Gothenburg, Sweden. == NEWS == * Lunches and at least one dinner for the Hackathon attendees will be provided through our generous sponsors Credit Suisse and Galois. * If you plan to start a new project on community.haskell.org, please request an account and project in advance, so as to save valuable Hackathon time. See http://community.haskell.org/admin/ for more details. == What it is == The plan is to hack on Haskell infrastructure, tools, libraries and compilers. To attend please register, and get ready to hack those lambdas! Code to hack on: * Hackage * Cabal * Porting foreign libraries * Bug squashing * You decide! Before you attend, do start thinking and familiarizing yourself with 1 or 2 projects you wish to work on, to ensure no wasted effort during the Hackathon. A list of possible projects is available on the website. == Registration == We ask that you register your interest. Follow the instructions on the registration page: http://haskell.org/haskellwiki/Hac4/Register Once you've registered, do add your info to the attendees self-organizing page, http://haskell.org/haskellwiki/Hac4/Attendees if you are looking to share costs, or meet up prior to the hackathon, with other attendees. N.B. if you already expressed interest via the wiki, do confirm by registering `officially' anyway. == Important dates == Hackathon: April 11-13, 2008 == Organizers == Björn Bringert (local) Thomas Schilling (local) Lennart Kolmodin (local) Krasimir Angelov (local) Jean-Philippe Bernardy (local) Ian Lynagh Duncan Coutts

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Re: [Haskell-cafe] simple Haskell question - List comprehensions
by Philip Müller 12 Mar '08

12 Mar '08

Dan Licata schrieb: > Does that help? > Yeah, it did - Thanks! - Philip

1 0

Exception handling when using STUArray
by oleg＠okmij.org 11 Mar '08

11 Mar '08

Sterling Clover wrote: > there's no standard way that I know of besides "inspection" to > determine if code might throw an exception, and this is particularly > the case with the dreaded lazy IO of prelude functions. The following old message showed even two ways of doing exactly that -- in Haskell, with no external tools. Exceptions in types and exception-free programming http://www.haskell.org/pipermail/haskell/2004-June/014271.html The third way described in the message -- ensuring that exceptions will never occur -- was further developed as lightweight static capabilities. > And as per a recent discussion on libraries, it's still a royal pain > to catch IO exceptions in monads with an IO base. That is again very easy. Takusen uses a CaughtMonadIO -- which includes a wide range of monads: IO itself and various transformations of it. I am extensively using a variant written by Jesse Tov based on the Takusen's idea. His code defines two classes: EMonad and EMonadIO -- which enclose most of the interesting monads. The latter is the subclass of the former and also allows arbitrary IO (via liftIO). In either case, I use gthrow, gbracket, gcatch, ghandle, gfinally, etc. -- without even thinking which Monad I'm in and how error handling is actually implemented (via ErrorT or via IO exceptions). It works universally for most of monads of interest.

1 0

Reflective capabilities of Haskell
by Martin Hofmann 11 Mar '08

11 Mar '08

I am trying to port a programme written in Maude, which is a reflective language based on rewriting logic ( http://maude.cs.uiuc.edu/ ), to Haskell. I thought using Template Haskell might be a good idea, but I got stuck and now I am wondering if this is really possible in Haskell. Let me give an example of a Maude module defining the function last over a list of Peano numbers. fmod LAST is sorts Peano PeanoList . op 0 : -> Peano [ctor] . op s : Peano -> Peano [ctor] . op [] : -> PeanoList [ctor] . op cons : Peano PeanoList -> PeanoList [ctor] . op last : PeanoList -> Peano . vars X Y : Peano . var Xs : PeanoList . eq last(cons(X,[])) = X . eq last(cons(X,cons(Y,Xs))) = last(cons(Y,Xs)) . endfm So, last(cons(s(0),cons(s(s(0)),cons(s(s(s(0))),[])))) would reduce to s(s(s(0))). The cool thing about Maude is, that terms, modules, ... can be lifted to the meta level. For example: upModule('LAST, false) would yield

1 0

Instance selection based on a class constraint [was: Issues(Bugs?) with GHC Type Families]
by oleg＠okmij.org 11 Mar '08

11 Mar '08

> Manuel M T Chakravarty: > Hugo Pacheco: > > I would simply like the compiler not to use that instance if the > > equality constraint does not hold, like some another instance > > dependency constraint, but I assume that is not possible. > > This is independent of type families. The selection of type class > instances does *never* dependent on the instance context, only on the > instance head. I know that this is sometimes annoying, but type > checking would be a a lot more complicated/expensive without that rule. What Hugo Pacheco wants *is* possible, in GHC as it is (and even as it was back in 2004). Although the selection of type class instances indeed depends only on the instance head, on the surface, the dependence on context is quite easy to accomplish. Here's is the outline of the idea. Suppose we wish to select one of the two instances, depending on whether the constraint C x hold or not: instance C x => CMain x instance CMain x We encode this as follows: class CMain x instance (CPred x flag, CMain' flag x) => CMain x The main class has only one instance, and there is no longer any overlapping. The main selection is done in an auxiliary class class CMain' flag x instance CMain' HTrue x -- will be selected when CPred holds instance CMain' HFalse x -- otherwise Note there are no overlapping instances required. The trick is to re-write constraint C x into the form CPred x flag (where flag functionally depends on x). Whereas C x succeeds, CPred x flag should succeed and unify flag with HTrue. Should C x fail, CPred should still succeed and unify flag with HFalse. TypeEq is such a predicate in the case of type equality. CPred is easy to implement with type-classes, and probably even easier with type families. The HList and OOHaskell papers use this technique to great extent. The following message describes that even backtracking is possible: http://okmij.org/ftp/Haskell/poly2.txt

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Where does ~> come from?
by Steve Lihn 11 Mar '08

11 Mar '08

While I am reading TypeCompose module, I am having trouble finding where ~> comes from? (And its son ~~> and grandson ~~~>, etc.) This is my immediate question. Help is appreciated. A bigger (higher-order?) issue I encountered and I think other people could also have is to look up all these special "symbols" during the learning curve. As things are getting higher and higher order in Haskell, more inventive symbols are appearing. It becomes harder to remember the exact definitions are. The difficulties are: 1) I used to see a page on haskell.org that lists many symbols, but I could not find that page any more. 2) You would hope there is a quick way to search those symbols. But most search engines do not treate symbols friendly, often just ignore them. I typed ~> in Hoogle, it also returned nothing. 3) If the module defining the symbol is not in standard library, it is not possible to look up the symbol in the core library index. The only way to find the symbol seems to go to the source code, check every module being imported and if I am lucky I will find it in the same package. If I am not lucky, I will have to search that module on the Internet, and hunt it down recursively. I wonder if anybody has the same problem. If enough crowd have the same problem, maybe there could be a better way to handle this kind of documentation/learning issue! Thanks, Steve

5 4

automagically?
by Galchin Vasili 11 Mar '08

11 Mar '08

Hello, Say i am building package X that depends on A, H, D. How can I automagically check to A, H, D need to "darcs get" down and built/install before X? If this feature is not available it should be, i.e. an integration of darcs and cabal. Regards, Vasya

3 2

concurrent definitions with parsec
by Zsolt SZALAI 10 Mar '08

10 Mar '08

Hi! I'm writing a parser for a language, with a BNF like this: Type = "type" Def Def = RecordDef | RecordOfDef ... RecordDef = "record" Body RecordOfDef = "record" "of" With a perser what uses parsec module it can be mapped to haskell easily: structuredTypeDef = recordDef <|> recordOfDef But this way, the recordOfDef case will never be parsed at a line like "type record <somethings>", because recordDef will win in the first place always. Is there an option or an other operation instead of <|> to do the trick? I cant see if the problem can be solved with parsecperm or not. It is a possibility to do something like swap "recordDef <|> recordOfDef" with "recordlikeDefs " where recordlikeDefs = do { reserved "record" ; others } but that would be the dirty way... Do you have any advise on the case? Thanks, -- Zsolt Szalai

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Issues(Bugs?) with GHC Type Families
by Hugo Pacheco 10 Mar '08

10 Mar '08

Hi all, I have recently tried to replicate some examples from in the articles about type families but found some possible bugs. In [2], the example class C a where type S a (k :: * -> *) :: * instance C [a] where type S [a] k = (a,k a) does not compile under the claim that the type variable k is not in scope. However, if we extract the type family from the class type family S a (k :: * -> *) :: * type instance S [a] k = (a, k a) class C a it compiles correctly. According to [3], the difference is purely syntactic sugar, does that mean that both examples should compile and behave the same or is there some subtlety that justifies the class example not to compile? Another issue is that data kinds (used in both [2] and [3]) do not seem to be supported at all by the compiler, are they already implemented in GHC? Simple examples such as datakind Nat = Zero or datakind Nat = Zero | Succ Nat fail to compile. Perhaps some of these should be submitted to the GHC Bug Tracker. I have tested both GHC 6.8.2 and 6.9.20080218. References: 1. Associated Types with Class.<http://www.cse.unsw.edu.au/~chak/papers/CKPM05.html> Manuel M. T. Chakravarty, Gabriele Keller, Simon Peyton Jones, and Simon Marlow. In *Proceedings of The 32nd Annual ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages (POPL'05)*, pages 1-13, ACM Press, 2005. 2. Associated Type Synonyms.<http://www.cse.unsw.edu.au/~chak/papers/CKP05.html> Manuel M. T. Chakravarty, Gabriele Keller, and Simon Peyton Jones. In *Proceedings of The Tenth ACM SIGPLAN International Conference on Functional Programming *, ACM Press, pages 241-253, 2005. 3. Towards Open Type Functions for Haskell.<http://www.cse.unsw.edu.au/~chak/papers/SSPC07.html> Tom Schrijvers, Martin Sulzmann, Simon Peyton-Jones, and Manuel M. T. Chakravarty. Unpublished manuscript, 2007.

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Haskell Weekly News: March 09, 2008
by Don Stewart 10 Mar '08

10 Mar '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080309 Issue 71 - March 09, 2008 --------------------------------------------------------------------------- Welcome to issue 71 of HWN, a newsletter covering developments in the [1]Haskell community. Another busy week on the Haskell library front, with around 100 new and updated libraries and tools on Hackage. 1. http://haskell.org/ Announcements Google Summer of Code. Malcolm Wallace [2]announced Google is running its 'Summer of Code' project again this year, and Haskell.org is once again going to apply to be a mentoring organisation. If you're interested in earning money to hack on Haskell, and helping out the community, [3]take a look at the wiki. 2. http://article.gmane.org/gmane.comp.lang.haskell.cafe/37273 3. http://hackage.haskell.org/trac/summer-of-code Haskell in the browser. Dimitry Golubovsky [4]announced that the YHC JavaScript backend is now in alpha testing, and is open to experimentation for those wanting to write Haskell directly for the browser 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/37299 Hackage New and updated libraries in [5]the Hackage library database. 5. http://hackage.haskell.org/ * typalyze 0.1.1. Uploaded by Matthew Danish. [6]typalyze: Analyzes Haskell source files for easy reference. * lax 0.0.0.1. Uploaded by Wolfgang Jeltsch. [7]lax: Lax arrows. * truelevel 0.1.1. Uploaded by Barton Massey. [8]truelevel: Audio file compressor-limiter. * WAVE 0.1. Uploaded by Barton Massey. [9]WAVE: WAVE audio file IO library. * parseargs 0.1. Uploaded by Barton Massey. [10]parseargs: Command-line argument parsing library for Haskell programs. * conjure 0.1. Uploaded by Gwern Branwen. [11]conjure: A BitTorrent client. * Diff 0.1.1. Uploaded by Sterling Clover. [12]Diff: O(ND) diff algorithm in haskell.. * simseq 0.0. Uploaded by Gwern Branwen. [13]simseq: Simulate sequencing with different models for priming and errors. * rbr 0.8.3. Uploaded by Gwern Branwen. [14]rbr: Mask nucleotide (EST) sequences in Fasta format. * xml2x 0.2. Uploaded by Gwern Branwen. [15]xml2x: Convert BLAST output in XML format to CSV or HTML. 6. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/typalyze-0.1.1 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lax-0.0.0.1 8. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/truelevel-0.1.1 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/WAVE-0.1 10. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parseargs-0.1 11. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/conjure-0.1 12. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Diff-0.1.1 13. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/simseq-0.0 14. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/rbr-0.8.3 15. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xml2x-0.2 * estreps 0.1. Uploaded by Gwern Branwen. [16]estreps: Repeats from ESTs. * clustertools 0.1. Uploaded by Gwern Branwen. [17]clustertools: Tools for manipulating sequence clusters. * xsact 1.6. Uploaded by Gwern Branwen. [18]xsact: Cluster EST sequences. * HsJudy 0.2. Uploaded by Gwern Branwen. [19]HsJudy: Judy bindings, and some nice APIs. * prof2dot 0.3.1. Uploaded by Gregory Wright. [20]prof2dot: Convert GHC profiles into GraphViz's dot format. * strict 0.3.2. Uploaded by Roman Leshchinskiy. [21]strict: Strict data types and String IO.. * Emping 0.4. Uploaded by Hans VanThiel. [22]Emping: derives heuristic rules from nominal data. * GuiHaskell 0.1.1. Uploaded by Neil Mitchell. [23]GuiHaskell: A graphical REPL and development environment for Haskell. * simpleargs 0.1. Uploaded by Ketil Malde. [24]simpleargs: Provides a more flexible getArgs function with better error reporting.. * parsec 3.0.0. Uploaded by Derek Elkins. [25]parsec: Monadic parser combinators. 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/estreps-0.1 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/clustertools-0.1 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xsact-1.6 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HsJudy-0.2 20. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/prof2dot-0.3.1 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/strict-0.3.2 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Emping-0.4 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GuiHaskell-0.1.1 24. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/simpleargs-0.1 25. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parsec-3.0.0 * hetris 0.2. Uploaded by Gwern Branwen. [26]hetris: Text Tetris. * hscurses 1.3. Uploaded by Gwern Branwen. [27]hscurses: NCurses bindings for Haskell. * photoname 2.0. Uploaded by Dino Morelli. [28]photoname: Rename JPEG photo files based on shoot date. * mage 1.1.0. Uploaded by Gwern Branwen. [29]mage: Rogue-like. * infix 0.1.1. Uploaded by Gwern Branwen. [30]infix: Infix expression re-parsing (for HsParser library). * bio 0.3.3. Uploaded by Gwern Branwen. [31]bio: A bioinformatics library. * dephd 0.0. Uploaded by Gwern Branwen. [32]dephd: Analyze 'phred' output (.phd files). * hybrid 2.0. Uploaded by Gwern Branwen. [33]hybrid: A implementation of a type-checker for Lambda-H. * propgrid 0.1. Uploaded by Gwern Branwen. [34]propgrid: GUI propertygrid. * gravatar 0.3. Uploaded by Donald Stewart. [35]gravatar: Find the url of the gravatar associated with an email address.. 26. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hetris-0.2 27. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hscurses-1.3 28. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/photoname-2.0 29. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mage-1.1.0 30. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/infix-0.1.1 31. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bio-0.3.3 32. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dephd-0.0 33. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hybrid-2.0 34. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/propgrid-0.1 35. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/gravatar-0.3 * himerge 0.17.9. Uploaded by Luis Araujo. [36]himerge: Haskell Graphical User Interface for Emerge. * Takusen 0.8. Uploaded by Alistair Bayley. [37]Takusen: Database library with left-fold interface, for PostgreSQL, Oracle, SQLite, ODBC.. * irc 0.4.1. Uploaded by Trevor Elliott. [38]irc: A small library for parsing IRC messages.. * hexpat 0.2. Uploaded by Evan Martin. [39]hexpat: wrapper for expat, the fast XML parser. * microbench 0.1. Uploaded by Evan Martin. [40]microbench: Microbenchmark Haskell code. * hxt 7.5. Uploaded by Uwe Schmidt. [41]hxt: A collection of tools for processing XML with Haskell.. * hmatrix 0.2.1.0. Uploaded by Alberto Ruiz. [42]hmatrix: Linear algebra and numerical computations. * binary-strict 0.3.1. Uploaded by Adam Langley. [43]binary-strict: Binary deserialisation using strict ByteStrings. * category-extras 0.1. Uploaded by Dan Doel. [44]category-extras: Various modules and constructs inspired by category theory.. * pcap 0.4.3. Uploaded by Bryan OSullivan. [45]pcap: A system-independent interface for user-level packet capture. 36. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/himerge-0.17.9 37. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Takusen-0.8 38. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/irc-0.4.1 39. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hexpat-0.2 40. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/microbench-0.1 41. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hxt-7.5 42. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hmatrix-0.2.1.0 43. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-strict-0.… 44. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/category-extras-… 45. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pcap-0.4.3 * curl 1.3.1. Uploaded by Eric Mertens. [46]curl: Haskell binding to libcurl. * fastcgi 3001.0.2. Uploaded by Bjorn Bringert. [47]fastcgi: A Haskell library for writing FastCGI programs. * hslogger 1.0.5. Uploaded by John Goerzen. [48]hslogger: Versatile logging framework. * HAppS-Server 0.9.2.1. Uploaded by David Himmelstrup. [49]HAppS-Server: Web related tools and services.. * HAppS-IxSet 0.9.2.1. Uploaded by David Himmelstrup. [50]HAppS-IxSet: Added by DavidHimmelstrup, Fri Feb 29 07:27:13 PST 2008.. * HAppS-State 0.9.2.1. Uploaded by David Himmelstrup. [51]HAppS-State: Event-based distributed state.. * HAppS-Data 0.9.2.1. Uploaded by David Himmelstrup. [52]HAppS-Data: HAppS data manipulation libraries. * HAppS-Util 0.9.2.1. Uploaded by David Himmelstrup. [53]HAppS-Util: Web framework. * sessions 2008.2.28. Uploaded by Matthew Sackman. [54]sessions: Session Types for Haskell. * utf8-string 0.3. Uploaded by Eric Mertens. [55]utf8-string: Support for reading and writing UTF8 Strings. 46. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/curl-1.3.1 47. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/fastcgi-3001.0.2 48. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hslogger-1.0.5 49. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-Server-0.9… 50. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-IxSet-0.9.… 51. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-State-0.9.… 52. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-Data-0.9.2… 53. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-Util-0.9.2… 54. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sessions-2008.2.… 55. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/utf8-string-0.3 * EdisonCore 1.2.1.2. Uploaded by Robert Dockins. [56]EdisonCore: A library of efficent, purely-functional data structures (Core Implementations). * parameterized-data 0.1. Uploaded by Alfonso Acosta. [57]parameterized-data: Parameterized data library implementing lightweight dependent types. * unix 2.3.0.0. Uploaded by Ross Paterson. [58]unix: POSIX functionality. * hoogle 3.1. Uploaded by Neil Mitchell. [59]hoogle: Haskell API Search. * ftshell 0.2. Uploaded by Janis Voigtlaender. [60]ftshell: Shell interface to the FreeTheorems library.. * free-theorems 0.2. Uploaded by Janis Voigtlaender. [61]free-theorems: Automatic generation of free theorems.. * special-functors 1.0. Uploaded by Henning Thielemann. [62]special-functors: Control.Applicative, Data.Foldable, Data.Traversable (compatibility package). * type-level 0.1. Uploaded by Alfonso Acosta. [63]type-level: Type-level programming library. * nymphaea 0.2. Uploaded by Gwern Branwen. [64]nymphaea: An interactive GUI for manipulating L-systems. * hsc3 0.2. Uploaded by Rohan Drape. [65]hsc3: Haskell SuperCollider. 56. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/EdisonCore-1.2.1… 57. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parameterized-da… 58. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/unix-2.3.0.0 59. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hoogle-3.1 60. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ftshell-0.2 61. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/free-theorems-0.2 62. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/special-functors… 63. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/type-level-0.1 64. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/nymphaea-0.2 65. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsc3-0.2 * hosc 0.2. Uploaded by Rohan Drape. [66]hosc: Haskell Open Sound Control. * hslackbuilder 0.0.1. Uploaded by Andrea Rossato. [67]hslackbuilder: HSlackBuilder automatically generates slackBuild scripts from a cabal package. * hsparklines 0.1.0. Uploaded by Hitesh Jasani. [68]hsparklines: Sparklines for Haskell. * sat-micro-hs 0.1.1. Uploaded by Denis Bueno. [69]sat-micro-hs: A minimal SAT solver. * interlude 0.1.1. Uploaded by Gwern Branwen. [70]interlude: Replaces some Prelude functions for enhanced error reporting. * parse-dimacs 1.0.1. Uploaded by Denis Bueno. [71]parse-dimacs: DIMACS CNF parser library. * bitset 0.5. Uploaded by Denis Bueno. [72]bitset: A functional data structure for efficient membership testing.. * special-functors 1.0. Uploaded by Henning Thielemann. [73]special-functors: Control.Applicative, Data.Foldable, Data.Traversable (compatibility package). * condorcet 0.0.1. Uploaded by Gwern Branwen. [74]condorcet: Library for Condorcet voting. * heap 0.2.3. Uploaded by Stephan Friedrichs. [75]heap: Heaps in Haskell. 66. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hosc-0.2 67. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hslackbuilder-0.… 68. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsparklines-0.1.0 69. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sat-micro-hs-0.1… 70. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/interlude-0.1.1 71. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parse-dimacs-1.0… 72. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bitset-0.5 73. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/special-functors… 74. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/condorcet-0.0.1 75. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/heap-0.2.3 * hspr-sh 0.3. Uploaded by Gwern Branwen. [76]hspr-sh: Session handler for HSP. * hsp 0.2. Uploaded by Gwern Branwen. [77]hsp: Haskell Server Pages is a library for writing dynamic server-side web pages.. * trhsx 0.2.1. Uploaded by Gwern Branwen. [78]trhsx: trhsx is the preprocessor for Harp and HSP. * haskell-src-exts 0.2.1. Uploaded by Gwern Branwen. [79]haskell-src-exts: Manipulating Haskell source: abstract syntax, lexer, parser, and pretty-printer. * harp 0.2.1. Uploaded by Gwern Branwen. [80]harp: HaRP allows pattern-matching with regular expressions. * HTF 0.1. Uploaded by Gwern Branwen. [81]HTF: The Haskell Test Framework. * hsdip 0.1. Uploaded by Gwern Branwen. [82]hsdip: hsdip - a Diplomacy parser/renderer. * mpdmate 0.1. Uploaded by Gwern Branwen. [83]mpdmate: MPD/PowerMate executable. * powermate 0.1. Uploaded by Gwern Branwen. [84]powermate: PowerMate bindings. * syb-with-class 0.4. Uploaded by David Himmelstrup. [85]syb-with-class: Scrap Your Boilerplate With Class. 76. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hspr-sh-0.3 77. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsp-0.2 78. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/trhsx-0.2.1 79. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haskell-src-exts… 80. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/harp-0.2.1 81. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTF-0.1 82. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsdip-0.1 83. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mpdmate-0.1 84. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/powermate-0.1 85. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/syb-with-class-0… * whim 0.1. Uploaded by Gwern Branwen. [86]whim: A Haskell window manager. * memcached 0.1. Uploaded by Gwern Branwen. [87]memcached: haskell bindings for memcached. * HaLeX 1.1. Uploaded by Gwern Branwen. [88]HaLeX: HaLeX enables modelling, manipulation and animation of regular languages. 86. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/whim-0.1 87. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/memcached-0.1 88. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HaLeX-1.1 Jobs Haskell for real-time control software. Tom Hawkins [89]announced an opening for a Haskell job in real-time control software for vehicle and machinery applications 89. http://article.gmane.org/gmane.comp.lang.haskell.cafe/37093 Haskell for bioinformatics. Ketil Malde [90]announced an open position for a 3-year Ph.D. scolarship at IMR working on bioinformatics projects in Haskell 90. http://news.gmane.org/gmane.comp.lang.haskell.cafe/cutoff=37362 Blog noise [91]Haskell news from the [92]blogosphere. * [93]Barracuda P2P Chat * [94]A Lambda Calculus Reducer * [95]A Fashion Magazine in Haskell * [96]Introduction to building stateful web apps in HAppS * [97]Intro to HAppS-State * [98]Project Euler in Haskell * [99]In praise of mandatory indentation for novice programmers * [100]More Monads on the Cheap: Inlined fromMaybe * [101]A First Haskell Experience * [102]Haskell and code coverage * [103]Why I don't use Haskell for Functional Programming (monads, lifting) 91. http://planet.haskell.org/ 92. http://haskell.org/haskellwiki/Blog_articles 93. http://haskell.org/gtk2hs/archives/2008/02/24/barracuda-p2p-chat/ 94. http://www.defmacro.org/ramblings/lambda-reducer.html 95. http://www.alpheccar.org/en/posts/show/91 96. http://softwaresimply.blogspot.com/2008/02/intro-to-happs-part-1.html 97. http://softwaresimply.blogspot.com/2008/02/intro-to-happs-state.html 98. http://extempore.livejournal.com/212602.html 99. http://okasaki.blogspot.com/2008/02/in-praise-of-mandatory-indentation-for.… 100. http://osteele.com/archives/2008/02/inlined-frommaybe 101. http://the-programmers-stone.com/2008/03/04/a-first-haskell-experience/ 102. http://dukedave.blogspot.com/2008/03/back-in-action.html 103. http://jlouisramblings.blogspot.com/2008/03/why-i-dont-use-haskell-for-func… Quotes of the Week * teamonkey: the Haskell solutions that people are posting are generally so much more concise and elegant than for any other language * Dan Zwell: I am fairly new to Haskell, and I didn't realize how easy concurrent code is until I wrote this * anonymous: The Haskall (sic) language is often uses by very intelligent programmers, it often allows to use lazy computations and iterations, but it has the advantage that its iterators behave better (than in Python), and during the generation of some items you can, when you want, refer and use the items already generated. * Corun: I don't understand, what's the advantage of hugs? The uni here says to use hugs, though, but I kept finding myself going in to ghci to get a useful error message * They say that if it compiles, it will run correctly. It?s nearly true! I?m amazed. ... Such buglessness will remove a huge source of indeterminism in production environments where the work of many teams is co-ordinated by schedules. * dolio: I've made a domain specific notation for describing puddings. * cschneid: [Haskell] changed the way I look at decomposition of problems in the more corporate languages (Java and C#). I use far fewer variables, and more side-effect free methods. It's made my code clearer, and easier to test. * nicodemus: I've written some Erlang and much more Haskell. My take so far is that Erlang is good for teaching you how to fish, Haskell is good for teaching you about procuring food (including fish). * paulzork: Haskell is to functional programming like C is to imperative languages? Sort of the latin root? About the Haskell Weekly News New editions are posted to [104]the Haskell mailing list as well as to [105]the Haskell Sequence and [106]Planet Haskell. [107]RSS is also available, and headlines appear on [108]haskell.org. Headlines are available as [109]PDF. To help create new editions of this newsletter, please see the [110]contributing information. Send stories to dons at galois.com. The darcs repository is available at darcs get [111]http://code.haskell.org/~dons/code/hwn/ 104. http://www.haskell.org/mailman/listinfo/haskell 105. http://sequence.complete.org/ 106. http://planet.haskell.org/ 107. http://sequence.complete.org/node/feed 108. http://haskell.org/ 109. http://code.haskell.org/~dons/code/hwn/archives/20080309.pdf 110. http://haskell.org/haskellwiki/HWN 111. http://code.haskell.org/~dons/code/hwn/

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simple Haskell question - List comprehensions
by Philip Müller 10 Mar '08

10 Mar '08

Hi, I'm just working through Hutton's "Programming in Haskell" and I found an exercise which I can't solve, although it looks simple. Maybe someone here could give me a hint? Exercise: Show how the single comprehension [(x,y) | x <- [1,2,3], y <- [4,5,6]] with two generators can be re-expressed using two comprehensions with single generators. Hint: make use of the library function _concat_. Thank you very much, Philip

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deriving instances of Enum for tuples of bounded, enumerable types (useful for generating QuickCheck.Arbitrary instances for collection of collection types)
by Thomas Hartman 09 Mar '08

09 Mar '08

I have some code for creating instances of Enum for a tuple of bounded enumerable types. The main win with this was that it makes it easier for me to generate Test.QuickCheck.Arbitrary instances for types that are based on collections of enumerable collections -- like you might get when modeling poker for instance. In fact, I was trying to answer the question about using QuickCheck with poker, that Vincent Foley asked a few days ago at http://groups.google.com/group/fa.haskell/browse_thread/thread/8298c4d838c9… when I came up with this. (And I am posting a reply to Vincent right after I finish writing this message.) Basically I noticed that coming up with the Arbitrary instances seemed to be tricker than I would have expected. However, it was pretty straightforward to write an instance of arbitrary for instance (Enum a, Bounded a) => Arbitrary a where arbitrary = do n <- choose (fromEnum (minBound :: a), fromEnum (maxBound :: a) ) return $ toEnum n So all I needed to do was get my type into instances of Enum and Bounded, and I got my Arbitrary instance for free. Most of the work was just writing the instances for various tuples of Bounded Enumerable types. Maybe I've been spoiled by how easy many types are gotten using the magic of the deriving clause, but this seemed like a lot of work to me. I was wondering if there was an easier way? Or if not, if it would make sense for something like this to be added to the prelude? Or is there some sizeable collection of nice Arbitrary instances for QuickCheck somewheres that I can leverage off of, that hasn't made it into the standard libraries? A minor issue: I had a question if I could make the type signatures for Enum instances less verbose by doing something like type BoundedEnum = (Bounded a, Enum a) => a... I tried, and commented out my attempt as it wouldn't type check. Guidance appreciated. Thanks, Thomas. Code follows: {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} module EnumInstances where -- instances of Enum for tuples of types that are both Bounded and Enum -- typechecks, and does the right thing, though this signature seems messy, specially when I -- want to do this kind of thing for longer tuples instance (Enum a, Enum b,Bounded a, Bounded b) => Enum (a,b) where fromEnum (primero,segundo) = ( divFactor * (fromEnum primero) ) + (fromEnum segundo) where divFactor = fromEnum (maxBound :: b) + 1 toEnum i | (i > (fromEnum (maxBound :: (a,b)))) || (i < (fromEnum (minBound :: (a,b)))) = error "Enum (a,b) bounds error" | otherwise = (a,b) where a = toEnum baseline b = toEnum $ i - (baseline * divFactor ) baseline = i `div` divFactor divFactor = (fromEnum (maxBound :: b)) + 1 -- I tried to use a type synonym to write a cleaner type syg but... type BoundedEnumerable = (Enum a, Bounded a) => a -- does not typecheck. oh well, the verbose signatures aren't all that bad. {- instance Enum (BoundedEnumerable,BoundedEnumerable) where fromEnum (primero,segundo) = ( divFactor * (fromEnum primero) ) + (fromEnum segundo) where divFactor = fromEnum (maxBound :: b) + 1 toEnum i | (i > (fromEnum (maxBound :: (a,b)))) || (i < (fromEnum (minBound :: (a,b)))) = error "Enum (a,b) bounds error" | otherwise = (a,b) where a = toEnum baseline b = toEnum $ i - (baseline * divFactor ) baseline = i `div` divFactor divFactor = (fromEnum (maxBound :: b)) + 1 -} instance (Enum t, Enum t1, Enum t2, Bounded t, Bounded t1, Bounded t2) => Enum (t,t1,t2) where fromEnum (a,b,c) = fromEnum ((a,b),c) toEnum i = (a,b,c) where ((a,b),c) = toEnum i instance (Enum t, Enum t1, Enum t2, Enum t3, Bounded t, Bounded t1, Bounded t2, Bounded t3) => Enum (t,t1,t2,t3) where fromEnum (a,b,c,d) = fromEnum ((a,b,c),d) toEnum i = (a,b,c,d) where ((a,b,c),d) = toEnum i instance (Enum t, Enum t1, Enum t2, Enum t3, Enum t4, Bounded t, Bounded t1, Bounded t2, Bounded t3, Bounded t4) => Enum (t,t1,t2,t3,t4) where fromEnum (a,b,c,d,e) = fromEnum ((a,b,c,d),e) toEnum i = (a,b,c,d,e) where ((a,b,c,d),e) = toEnum i

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c_open and c_close signatures in the unix package
by Galchin Vasili 09 Mar '08

09 Mar '08

Hello, I need to find the c_open and c_close signatures that are referenced in unix/System/Posix/IO.hsc. Thanks, Vasya

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reply to vincent foley's question about poker
by Thomas Hartman 09 Mar '08

09 Mar '08

Weirdly, I came across a question Vincent Foley asked in FA Haskell when I read it in the googlegroups interface, http://groups.google.com/group/fa.haskell/browse_thread/thread/8298c4d838c9… but I don't seem to have his question in my inbox where I get haskell cafe. Maybe it got filtered somehow? I notice nobody else from Haskell Cafe seems to have answered, and this is usually a pretty helpful place so I am thinking maybe it really did get filtered. Oh well, I will answer it anyway now.. The question had to do with writing Arbitrary instances for datatypes useful for playing poker, and using quickCheck with them. I like my solution because I get Arbitrary instances for basically every datatype that's involved, from a single definition that gives you Arbitrary for any type a that is both Enum and Bounded. instance (Enum a, Bounded a) => Arbitrary a where arbitrary = do n <- choose (fromEnum (minBound :: a), fromEnum (maxBound :: a) ) return $ toEnum n Here's the code. (Note: For the code to compile you need the EnumInstances .hs module. I posted this a few minutes ago on haskell cafe.) {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} module Cards where import Test.QuickCheck import Control.Arrow import Debug.Trace import EnumInstances -- Types data Suit = Clubs | Diamond | Heart | Spade deriving (Show, Eq, Bounded, Enum, Ord) data Rank = Ace | Two | Three | Four | Five | Six | Seven | Eight | Nine | Ten | Jack | Queen | King deriving (Show, Eq, Bounded, Enum,Ord) --instance Arbitrary Rank where -- arbitrary = return $ choose data Card = Card (Rank, Suit) deriving (Eq,Show,Bounded, Ord) instance Enum Card where fromEnum (Card (r,s)) = fromEnum (r,s) toEnum i = Card (toEnum i) -- to check a new instance with quickCheck -- quickCheck (pEnum :: SomeType -> Bool) pEnum x = x == (toEnum . fromEnum) x -- type Hand = [Card] -- a hand is five cards data Hand = Hand Card Card Card Card Card deriving (Eq,Show, Bounded) instance Enum Hand where fromEnum (Hand a b c d e) = fromEnum (a,b,c,d,e) toEnum i = Hand a b c d e where (a,b,c,d,e) = toEnum i instance (Enum a, Bounded a) => Arbitrary a where arbitrary = do n <- choose (fromEnum (minBound :: a), fromEnum (maxBound :: a) ) return $ toEnum n -- not a very useful property, and certainly fails -- but it proves we have a working Arbitrary instance for Hand. pHandIsAlwaysAceOfClubs h = h == (Hand c c c c c) where c = Card (Ace, Clubs) t1 = do quickCheck (pEnum :: Rank -> Bool) quickCheck (pEnum :: Suit -> Bool) quickCheck (pEnum :: Card -> Bool) quickCheck (pEnum :: Hand -> Bool) quickCheck pHandIsAlwaysAceOfClubs -- of course this fails traceIt x = trace (show x) x

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Difficulties implementing an incremental parser using Oleg-style left fold enumerator
by Johan Tibell 09 Mar '08

09 Mar '08

Dear haskell-cafe, I'm trying to write a parser combinator library with the following contraints: * Parses LL(1) grammars. * Is incremental i.e. it uses an Oleg style left fold enumerator to receive its input. * Is applicative but not monadic. The problem -- maybe there are others too -- is that when a parser such as many (byte 65) is run it will always return a 'Partial' result waiting for more input even though the enumerator is exhausted. In other words, there's no way to detect end of input. My current implementation of the parser type is newtype Parser r a = Parser { unParser :: S -> (a -> S -> Result r) -> (S -> Result r) -> Result r } where the first parameter is the parse state, the second a success continuation, and the third a failure continuation. The only tricky part (except for the above mentioned problem) is to implement the choice operator. I implement mine as instance Applicative (Parser r) where pure a = ... p <*> p' = Parser $ \s succ fail -> flip (unParser p s) fail $ \f s' -> unParser p' s' (succ . f) fail which I think is correct. Here follows my code. I hope someone has some idea how I could handle the end of input problem correctly. Thanks. -- Johan {-# LANGUAGE DeriveDataTypeable, Rank2Types #-} ----------------------------------------------------------------------------- -- | -- Module : Parsing.IParse -- Copyright : (c) Johan Tibell 2008 -- License : BSD3-style (see LICENSE) -- -- Maintainer : johan.tibell(a)gmail.com -- Stability : experimental -- Portability : portable -- -- An incremental LL(1) parser combinator library. -- ----------------------------------------------------------------------------- module Parsing.IParse ( -- * The 'Parser' type Parser, Enumerator, parse, -- * Primitive parsers satisfy, byte, module Control.Applicative ) where import Control.Applicative (Alternative(..), Applicative(..)) import Control.Monad (Functor(..)) import qualified Data.ByteString as S import Data.Int (Int64) import Data.Typeable (Typeable, showsTypeRep, typeOf) import Data.Word (Word8) import Prelude hiding (fail, succ) -- --------------------------------------------------------------------- -- The Parser type -- | The parse state. data S = S {-# UNPACK #-} !S.ByteString {-# UNPACK #-} !Int64 deriving Show -- | A parse either succeeds, fails or returns a suspension with which -- the parsing can be resumed. data Result a = Finished a S | Failed Int64 | Partial (S.ByteString -> Result a) deriving Typeable -- | For debug output. instance (Show a, Typeable a) => Show (Result a) where showsPrec d (Finished a s) = showParen (d > 10) showStr where showStr = showString "Finished " . showsPrec 11 a . showString " " . showsPrec 11 s showsPrec d (Failed pos) = showParen (d > 10) showStr where showStr = showString "Failed " . showsPrec 11 pos showsPrec d (Partial k) = showParen (d > 10) showStr where showStr = showString "Partial " . showsTypeRep (typeOf k) -- | A parser takes a parse state, a success continuation and a -- failure continuation and returns a 'Result'. newtype Parser r a = Parser { unParser :: S -> (a -> S -> Result r) -> (S -> Result r) -> Result r } -- --------------------------------------------------------------------- -- Instances instance Functor (Parser r) where fmap f p = Parser $ \s succ fail -> unParser p s (succ . f) fail instance Applicative (Parser r) where pure a = Parser $ \s succ _ -> succ a s p <*> p' = Parser $ \s succ fail -> flip (unParser p s) fail $ \f s' -> unParser p' s' (succ . f) fail instance Alternative (Parser r) where empty = Parser $ \s _ fail -> fail s p <|> p' = Parser $ \s@(S _ pos) succ fail -> unParser p s succ $ \s'@(S _ pos') -> if pos == pos' then unParser p' s' succ fail else fail s' -- --------------------------------------------------------------------- -- Running a parser -- | The initial, empty parse state. initState :: S initState = S S.empty 0 -- | This is the final continuation that turns a successful parse into -- a 'Result'. finishedCont :: a -> S -> Result a finishedCont v s = Finished v s -- | This is the final continuation that turns an unsuccessful parse -- into a 'Result'. failedCont :: S -> Result a failedCont (S _ pos) = Failed pos -- | A enumerator is a partially applied left fold over some -- 'S.ByteString' input. The caller supplies an initial seed and an -- iteratee function. The iteratee function returns @Left seed@ if it -- want to terminate the iteration early, otherwise @Right seed@. type Enumerator m s = (s -> S.ByteString -> Either s s) -> s -> m s -- | @parse p enumerator@ runs the parser @p@, pulling in new data -- using @enumerator@ when necessary, and return @Left pos@ on failure -- and @Right val remaining@ on success. parse :: Monad m => Parser r r -> (forall s. Enumerator m s) -> m (Either Int64 (r, S.ByteString)) parse p enumerator = -- First test if the parser can succeed without consuming any -- input. let seed = (unParser p) initState finishedCont failedCont in case seed of Failed pos -> return $ Left pos Finished x (S s _) -> return $ Right (x, s) _ -> -- Otherwise, use the enumerator to feed the parser some -- input. do (result, pos) <- enumerator iter (seed, 0) return $ case result of Failed pos' -> Left pos' Finished x (S s _) -> Right (x, s) Partial _ -> Left pos where iter (Partial k, pos) chunk = let pos' = pos + fromIntegral (S.length chunk) in case k chunk of partial@(Partial _) -> Right (partial, pos') result -> Left (result, pos') iter _ _ = error "Should be partial." -- --------------------------------------------------------------------- -- Primitive parsers -- | The parser @satisfy p@ succeeds for any character for which the -- supplied function @p@ returns 'True'. Returns the character that -- is actually parsed. satisfy :: (Word8 -> Bool) -> Parser r Word8 satisfy p = Parser $ \st@(S s pos) succ fail -> case S.uncons s of Just (b, bs) -> if p b then succ b (S bs (pos + 1)) else fail st Nothing -> Partial $ \s' -> unParser (satisfy p) (S s' pos) succ fail -- | @byte b@ parses a single byte @b@. Returns the parsed byte -- (i.e. @b@). byte :: Word8 -> Parser r Word8 byte b = satisfy (== b)

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[pdxfunc] pdxfunc meeting: Monday, March 10, 7pm, CubeSpace
by Justin Bailey 08 Mar '08

08 Mar '08

The Portland Functional Programming group is meeting again this Monday, March 10, at 7 p.m. Join us! ---------- Forwarded message ---------- From: Igal Koshevoy <igal(a)pragmaticraft.com> Date: Fri, Mar 7, 2008 at 11:26 AM Subject: [pdxfunc] pdxfunc meeting: Monday, March 10, 7pm, CubeSpace To: Igal Koshevoy <igal(a)pragmaticraft.com> Join us at the next meeting of pdxfunc, the Portland Functional Programming Study Group. We'll have presentations, demos and discussions. We welcome programmers interested in all functional languages and our meetings have content for coders of all skill levels. If interested, please also subscribe to our mailing list at http://groups.google.com/group/pdxfunc PRESENTATION * Dr. Bart Massey: Audio Processing With Haskell - Abstract: Dr. Massey will discuss and demonstrate a Haskell implementation of a WAVE file I/O library and compressor-limiter application that he's been working on. He'll also talk about why he chose Haskell to do this. - Speaker: Dr. Bart Massey teaches Computer Science at Portland State University, focusing on open source software engineering and artificial intelligence. He's the current Secretary of the X.org Board of Directors, co-author of the Nickle open source programming language, and spent many years as the faculty advisor the Portland State Aerospace Society that builds open source sounding rockets. REMAINDER We'll spend the rest of the meeting doing open discussion, talking about projects, technical challenges, recent developments, etc. It's a good group with a lot of varied experience, so there won't be any shortage of interesting discussion. NEWS Don Stewart, who gave the group's first presentation, recently announced that the Haskell-based xmonad tiling window manager he helps write is now one year old (http://www.haskell.org/pipermail/xmonad/2008-March/005021.html) and the "Real World Haskell" book that he, Bryan O'Sullivan and John Goerzen are writing has released some new draft chapters for previewing (http://book.realworldhaskell.org/) Congratulations, Don! Anyway, see you at the meeting! --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "pdxfunc" group. To post to this group, send email to pdxfunc(a)googlegroups.com To unsubscribe from this group, send email to pdxfunc-unsubscribe(a)googlegroups.com For more options, visit this group at http://groups.google.com/group/pdxfunc?hl=en -~----------~----~----~----~------~----~------~--~---

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Re: Data.HashTable
by gwern0＠gmail.com 08 Mar '08

08 Mar '08

On 2008.03.06 22:43:53 +0100, Johannes Waldmann <waldmann(a)imn.htwk-leipzig.de> scribbled 1.4K characters: > > In practice, Data.Map outperforms it in essentially all cases > > (Data.HashTable stops working beyond a certain size and so any > > asymptotic benefits, if they exist at all, don't have time to kick > > in). > > What! > > I learned at school, and I teach my students, > * balanced binary tree: costs are log(size) > * hashtable: cost are essentially constant > therefore, hashtable should be preferred in almost all cases > (if you know a reasonable hash function > and except where you need persistency, of course) > > the difference should be visible even for moderate sizes > (e.g. 1000). With a reasonable implementation I expect > log(1000) = 10 comparisons (and dereferencings) for the tree; > while the hashtable should have the computation of the hash value > (ideally, in registers), one memory access, and one comparison. > > ... but indeed some experiments with Data.Map and Data.Hashtable > support the point you made. That's either good for Data.Map > or bad for Data.Hashtable. > > PS: I did not manage to compile HsJudy-1.0 with ghc-6.8.2 > (some hsc file missing - perhaps that should be auto-generated? how?) > > Best regards, Johannes. Oh... I'm terribly sorry about that. It was I who uploaded HsJudy. The problem was, it's maintained by the Pugs project for some reason. The directory structure looks like: pugs/ thirdparty/ HsJudy/ hsregex/ HsSyck/ installed/ judy/ Judy-1.0.3/ Here Judy-1.0.3/ contains the actual C library Judy itself. So what the Cabalized package residing in HsJudy/ was doing was literally linking against stuff like '../judy/Judy-1.0.3/src/JudyL/*.o'. At the time I was packaging, Cabal didn't yet warn about problems like ../, so it would build and install just fine when I ran it with no problems; but I forgot that obviously all the ../ stuff would totally break in an sdist tarball. I think I've fixed it: <http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HsJudy-0.1.1>. -- gwern Air eavesdropping pipe-bomb TSCM Centro ^X JIC TWA USACIL Protection

1 0

Thanks for the help: image of draft visualization
by Jefferson Heard 08 Mar '08

08 Mar '08

Thanks for everyone's help on the list re my Haskell woes with the latest visualization effort. I've been making my code more generic for the last week, and I plan on releasing a visualization framework back to the community at some point. Gotta get approval from my boss before releasing code back to the wild, but here's an image of the draft of the visualization I've been working on: http://vizdata.renci.org/projects/jeff/ProteinViz/ProteinViz.png Basically, what you're seeing is 18,500 genes arranged in a full heirarchical clustering (the clustering technique uses a metric I'm unfamiliar with, and I got the dataset pre-packaged from the guy who's using it). The final visualization is fully interactive and runs fine under Linux, dies a miserable death under windows (runs out of RAM, and I can't figure out why, nor do I particularly care), and primarily runs on our 14 foot by 9 foot (4.5x3 metres for those of us in metric) linux display wall. The full program is about 800 linux of Haskell code using nothing but the standard library under GHC 6.8. Again, thanks for all your help! -- Jeff

2 1

Network.Socket Woes
by Scott Bell 08 Mar '08

08 Mar '08

Hi all, I'm trying to do some standard socket stuff using the Network.Socket library. This example is a UDP server: import Network.Socket proto_udp :: ProtocolNumber proto_udp = 17 port = 10162 main = do s <- socket AF_INET Datagram 17 bindSocket s (SockAddrInet (PortNum port) iNADDR_ANY) putStrLn $ "Listening on port " ++ (show port) ++ "... " (msg, n, addr) <- recvFrom s 1024 putStrLn $ "Received " ++ (show n) ++ " bytes from " ++ (show addr) ++ ": " putStrLn (show msg) Using the equivalent C program (socket, bind, recvfrom) I'm able to successfully receive a 64 byte message from my test program. The Haskell version, however, does not return from recvFrom. I've also tried wrapping this in a withSocketsDo, with no effect. How can I proceed to debug/troubleshoot this program? Thanks, - Scott Bell

7 9

Small displeasure with associated type synonyms
by Stefan Holdermans 07 Mar '08

07 Mar '08

Dear all, I was doing a little experimentation with associated type synonyms and ran into a small but unforeseen annoyance. The following, contrived, example snippets illustrate my pain: First, let us declare some of the simplest classes exhibiting associated type synonyms that I can think of, class C a where type T a val :: T a together with a trivial instance for nullary products: instance C () where type T () = () val = () But then, let us try an instance for binary products: instance (C a, C b) => C (a, b) where type T (a, b) = (T a, T b) val = (val, val) I really thought this would work out nicely, but GHC (version 6.8.2) gracefully gives me Couldn't match expected type `T a2' against inferred type `T a' Expected type: T (a2, b) Inferred type: (T a, T a1) In the expression: (val, val) In the definition of `val': val = (val, val) Couldn't match expected type `T b' against inferred type `T a1' Expected type: T (a2, b) Inferred type: (T a, T a1) In the expression: (val, val) In the definition of `val': val = (val, val) while I think I deserve better than that. Can someone (Tom?) please explain (a) why the required unifications fail, (b) whether or not it is reasonable to expect the unifications to succeed, and (c) how I can overcome problems like these? Surely, I can have val take a dummy argument, but I am hoping for something a bit more elegant here. I tried lexically scoped type variables, but to no avail: instance forall a b. (C a, C b) => C (a, b) where type T (a, b) = (T a, T b) val = (val :: T a, val :: T b) gives me Couldn't match expected type `T a2' against inferred type `T a' In the expression: val :: T a In the expression: (val :: T a, val :: T b) In the definition of `val': val = (val :: T a, val :: T b) etc. Cheers, Stefan

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Analysing Haskell with Graph Theory
by Ivan Miljenovic 07 Mar '08

07 Mar '08

Back in December, I posted that I was thinking about doing a project for my honours year on using graph theory for analysis on Haskell source code, amongst others: http://thread.gmane.org/gmane.comp.lang.haskell.cafe/32912 Well, I've officially started this project, and my supervisor and I have come up with the following functions that would probably be relevant to analysing _any_ piece of source code (the actual code will be tentatively stored as a directed graph with the nodes being functions and the edges being function calls, i.e. if f calls g, there is a directed edge from f to g): * root finding -> find nodes with no incoming edges (in a program, you'd ideally want only main to be such a node) * cycle detection -> find a possibly recursive cycle in your code: if the cycle is too big, then you may wish to consider refactoring * depth analysis -> find leaves that have a depth from the root node that is extremely large compared to the others (if a function is 50 calls down from main compared to an average of 15, you may wish to refactor) * chain detection -> find connected functions that have only one incoming and one outgoing edge, e.g. : f -> g -> h : if g isn't used anywhere else, you may wish to combine it inside either f or g * node popularity -> get a count of how many functions use a particular function and how many other functions it calls (related to chain detection above) * clique detection -> probably not as relevant to source code, but if you have a large number of functions that are all pairwise co-recursive than you may wish to refactor Can anyone think of any other kind of functions that would be useful in this kind of source code analysis? Also, I'm going to be using haskell-src for for the parsing of the Haskell code so that I don't waste my time writing a parser, when my project is technically on the graph-theory side of things. I know that it focuses mainly on H98 with only a few extensions... is this likely to be a problem if I want to try running my eventual program on say ghc or xmonad as test examples? -- Ivan Lazar Miljenovic

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Build dependency problem with bytestring.
by Jon Strait 07 Mar '08

07 Mar '08

Hi all, I'm upgrading HAppS from my 0.9.2 to the current 0.9.2.1 in Hackage. Some HAppS components have built and installed (IxSet and Util). Halfway through the HAppS-State build, the GHC runtime linker gives a fatal error on finding a duplicate definition for symbol fps_minimum while loading bytestring 0.9.0.4 after having already loaded bytestring 0.9.0.1. From trial and error, I'm guessing that HAppS-State needs 0.9.0.1 but one of its build dependencies was built with 0.9.0.4. Using GHC 6.8.2 Linux here. Any suggestions on how best to currently approach this situation? Thanks, Jon

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ANN: Emping 0.4
by Hans van Thiel 07 Mar '08

07 Mar '08

Hello All, Emping 0.4, a utility which can derive heuristic rules from facts in Open Office Calc spreadsheet .csv format, has been released on HackageDB. Version 0.4 supports blank fields, treated as 'none of the others', which allows for split attribute values. For example, "Owns Car" could have values "Yes" or 'No", and then "Yes" could be split into "Make", "Price Range" and so on, while the fields for "No" could be blank for these attributes. Data still have to be in the spreadsheet format of rows and columns (and in OO Calc .csv). Version 0.4 has a Gtk2Hs based GUI and the code has been changed in several places. Compiled with GHC 6.8 it appears to be noticeably faster than version 0.3. The package comes with a user guide, a white paper, and two data files from the UCI Machine Learning Repository. To get an impression see: http://j-van-thiel.speedlinq.nl/emp/empug.html The compiled binary for Linux, with documentation, can also be downloaded directly from my web site. Thank you, Hans van Thiel

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Re: [Haskell-cafe] Analysing Haskell with Graph Theory
by Ivan Miljenovic 07 Mar '08

07 Mar '08

(CC'd to the list) On 07/03/2008, Dusan Kolar <kolar(a)fit.vutbr.cz> wrote: > I'm just wondering what algorithms you use for analysis. For instance, > clique is an NP complete problem. Do you use some approximation? If yes, > which? I'm particularly interested in the algorithm. I haven't gotten that far yet ;-) As it stands, my supervisor have discussed this slightly and I'll probably start off with a more-or-less brute-force approach. This should be OK as the graph will probably be reasonably sparse and (hopefully) not too big! -- Ivan Lazar Miljenovic

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